Lately, a receiver capable of receiving radio signals transmitted from satellites has come to be mounted in mobiles such as cars and portable phones. It has then become possible to estimate position of the mobile carrying such receiver by positioning by means of GPS (Global Positioning System). The technology for estimating position by using such receiver is a common basic technology essential in a variety of fields such as navigation, security, entertainment and others. However, the position estimating technology based on positioning by means of the GPS has had problems that it takes a long time for synchronization acquisition in starting the positioning and that it is difficult to be used within a house or basement where the radio signals from the satellites do not reach.
The Patent Document 1 has disclosed a technology that enables PHS (Personal Handyphone System) to measure strength of signals transmitted from base stations to estimate its own position based on the strength of the measured signals. Specifically, communication providers place the base stations of the PHS, so that positions where they have been installed are normally known. Therefore, the PHS can estimate its own position by the principle of triangulation based on the positions of the respective base stations by measuring the strength of the signals transmitted from three or more base stations and by estimating distances between the respective base stations and its own position based on the strength of the measured signals.
It is also conceivable to provide a position estimating method that enables a radio communication apparatus that communicates wirelessly with base stations (access points) of a wireless LAN (Local Area Network) to measure strength of signals transmitted from the base stations to estimate the position of the radio communication apparatus based on the signal strength. For instance, the base station of the wireless LAN transmits beacons for announcing its existence to its surroundings at certain frequency (e.g., 5 times/sec.). The radio communication apparatus can estimate its own position based on the signal strength of the beacon and position of the base station of the wireless LAN stored in advance. This position estimating method enables the estimation of position even within a house or basement that has been difficult by the position estimating technology based on positioning by GPS because the base station of the wireless LAN may be installed within the house and basement. That is, the position estimating method described above may be a very convenient and simple position estimating technology, provided that the radio communication apparatus is arranged to store base station information indicating positions of the base stations of the wireless LAN installed around the radio communication apparatus.